The present invention relates to an improvement in a push type mechanical pencil having an inner cylinder integrally molded of a synthetic resin, the inner cylinder comprising a chuck portion for intermittently pushing out a lead in cooperation with a clamping ring slidably inserted between the front end of a barrel and a ferrule fixed to the front end of the barrel, and a spare leads containing tube portion communicating with the chuck portion.
Molding the inner cylinder of a synthetic resin is advantageous in that it can be made very easily and inexpensively as compared with a metallic inner cylinder made by combining several parts together. It is further advantageous in that it makes the assembly of the mechanical pencil easier and increases the reliability of the operation thereof.
A mechanism and operation of a conventional prior art push type mechanical pencil are as follows: secure retention of a lead to prevent it from retracting due to the writing pressure can be attained by chuck means cooperating with clamping means. Namely, an inner cylinder is biassed by a compression spring fitted in a barrel so as to engage the outer surface of conical members of a chuck portion with the inner surface of a clamping ring to cause the conical members surrounding the outer surface of the lead to squeeze inwardly, thereby pressing against the lead to retain the lead. The lead is pushed out a predetermined amount by pushing the rear end portion of the inner cylinder downwardly against the bias of the spring, so that the clamping ring with the chuck portion is advanced downwardly a predetermined distance, while tightly engaging the lead. When the inner cylinder, or the chuck portion is further advanced even after the clamping ring has been stopped by a stepped portion of a ferrule, the chuck portion releases the lead, and the conical members of the chuck portion are spread apart from one another due to their own elasticity. When the pushing force on the rear end portion of the inner cylinder is removed, the clamping ring is again engaged with the inner cylinder, or the spread conical members and is retracted by the force of the spring. At the time when the retracting movement of the clamping ring is stopped by a stepped portion of the barrel, the above-described firm engagement between the chuck portion and the lead is established again.
There is already known in the prior art an inner cylinder having a chuck portion molded of a synthetic resin consisting of several longitudinally separated structural units. For example, U.S. Pat. No. 3,945,733 discloses an embodiment which has an inner cylinder separated into two structural units, and Japanese Utility Model Laid-open Specification No. 51- 59642 discloses an embodiment which has an inner cylinder separated into three structural units. In both of these two cylinders, an annular projection is provided on the inner surface of the barrel and the compression spring is seated on one end surface of the annular projection and the clamping ring contacts the other end surface of the annular projection, and the separated structural units are then forced through the spring and the annular projection into the clamping ring one by one to assemble a complete inner cylinder. The thus assembled inner cylinder is held by the clamping ring since the outer surface of conical members of the chuck portion is engaged with the inner surface of the clamping ring, even when the inner cylinder is biased by the spring.
The prior art push type mechanical pencil of having the construction as above described causes some inconveniences which will be described below. First, the inner cylinder may slip out of the rear end opening of the barrel. The inner cylinder biased by the spring is prevented from slipping out of the barrel only by the engagement of the conical outer surface of the chuck portion with the inner surface of the clamping ring. Therefore, the easier the assembly of the inner cylinder by forcing the separated structural units into the clamping ring one by one, the more the possibility that the inner cylinder will slip out of the rear end opening of the barrel. It is, however, undesirable that an additional member be provided, for example, at the rear end of the barrel to prevent the inner cylinder from slipping out since the number of parts increases. Secondly, the separate structural units once assembled to form the complete inner cylinder may slide relative to each other during the use of the mechanical pencil. These several separate structural units are combined to form the inner cylinder merely by bundling them inside the clamping ring; they are not united, for example, by using an adhesive or the like. Therefore, for example, when only one specific structural unit of the inner cylinder is pushed by the end of the spring by chance, then only that unit may slide with respect to the other structural units. In such a case, it is inevitable that the structural unit which has thus moved causes some difficulty in the functioning of the mechanical pencil.